The present invention relates to a fire detector with at least two electrodes between which an electric field is produced by a DC voltage source, whereby the surfaces of the electrodes which face each other define, a measuring chamber through which the air to be monitored moves by convection, one of the electrodes being arranged as a measuring electrode and the other electrode as a counter-electrode. A current measuring device and evaluating circuit is also provided.
Similar types of fire detectors are known and in use in various forms. For example, U.S. Pat. No. 2,408,051 (Donelian) shows a fire detector with two measuring chambers, wherein in the first measuring chamber particles and small ions are "filtered out" and in the second measuring chamber the air is ionized by means of a radioactive substance. The conductivity produced in this manner is reduced in the presence of smoke because ions settle onto the heavy mobile particles of smoke. When the reduction of conductivity in the ionization chamber reaches a threshold value, an alarm is released. In the above cited patent document the electrodes which define the measuring chambers are connected in series between the positive and the negative poles of a DC voltage source as a capacitive voltage divider. Moreover, relatively small field intensities of about 40 or 50 Volt/cm are provided in the first measuring chamber. The fire detector according to the above cited patent requires the use of both measuring chambers. The second measuring chamber, provided with a radioactive substance, raises environmental concerns. Moreover, the device is of a costly construction.
U.S. Pat. No. 3,754,219 (Klein) shows an apparatus for investigating the pollution of air or smoke in which the net charge is measured. However, the net charge varies so strongly in case of fire that fire detector operation based on this principle is impractical. The same concerns apply to the fire and smoke detector described in U.S. Pat. No. 3,470,551 (Jaffe et al).
Ion measuring apparatus or measuring devices for investigating the mobility of particles are also known, see for example the device described in U.S. Pat. No. 4,104,088. However, the use of such apparatus as a fire detector has not previously been proposed. Moreover, such apparatus is not for fire detection because of its construction. Most existing fire detectors comprise an ionization chamber which use radioactive substances, although there has been a long felt need for a simple fire detector that is not a burden on the environment and that operates without radioactive substances. This follows also from the article of Scheidweiler in "Staub-Reinhalt-Luft, Vol. 32, No. 11, November 1972".